The present invention relates in general to a method of fabricating a semiconductor device and in particular concerns a method of fabricating a semiconductor device through selective diffusion of aluminum in a vapor phase into a silicon substrate at a predetermined region by using a silicon oxide film as a diffusion mask.
As is well known, diffusion of aluminum into the silicon substrate is carried out in a sealed refractory tube or enclosure such as of quartz in which the silicon substrate and an impurity source of aluminum are placed and heated. It is believed that aluminum vapor evaporated from the diffusion source will first react with the quartz of the sealed enclosure and subsequently aluminum atoms are driven into the silicon substrate with a high dopant concentration to a limit of the solid solubility. In this connection, it is also known that the silicon oxide film is utterly useless as the mask for the diffusion of aluminum with a high concentration, since the silicon oxide film is easily reduced by aluminum vapor. Such phenomenon has been already reported in detail by Y.C. Kao of Westinghouse Electric Corporation, Research And Development Center, Pittsburgh, Pa., in an article "On the Diffusion of Aluminium into Silicon" in a periodical "Electrochemical Technology" No. 3-4, pp. 90 to 94. According to the reports of studies heretofore made, the reduction of the silicon oxide film is explained by the fact that aluminum suboxide (Al.sub.2 O) which is more active than aluminum has a higher vapor pressure in liquefied aluminum and contributes not only to the diffusion of aluminum with a high concentration but also undesirably to the reduction of the silicon oxide film.
There are known a silicon nitride film, alumina film and other composite films as the masking film for the gaseous selective diffusion of aluminum. Among them all, the masking effect of the silicon nitride film is discussed in detail by Masami Yokozawa of Research Lab., Matushita Electronics Corp. in a periodical "National Technical Report" No. 6, 1968, under the title "Diffusion of Aluminium in Silicon By Sealed Quartz Tube Method," a first report of his studies on "Diffusion of Aluminium Into Silicon Crystals." However, there are few practical cases in which the silicon nitride film, alumina film and other composite films are actually employed as the mask for the selective diffusion of aluminum, because difficulties are encountered in the treatments such as photo-etching for forming such films, involving high manufacturing costs and possibility of formation of pin-holes and cracks.
In contrast, the silicon oxide film is immune to these drawbacks and is regarded as an ideal masking material. However, it has been hitherto impossible to use a silicon oxide film as the mask for the gaseous selective diffusion of aluminum for the reasons described above, that is, the reduction of the silicon oxide film by aluminum vapor.